1. Field of the Invention
A prosthetic knee joint is provided with a femoral component, a tibial component and a bearing between the femoral and tibial components. The bearing is capable of rotational movement on the tibial component and anterior-posterior sliding movement on the tibial component in response to flexion of the knee.
2. Description of the Related Art
U.S. Pat. No. 5,702,466 shows a knee prosthesis with a tibial component that has a superior bearing surface. The prosthesis further includes a femoral component with an inferior articular bearing surface. A bearing is disposed between the tibial and femoral component and includes an inferior surface in rotating and sliding bearing engagement with the superior surface of the tibial component. The bearing further includes a superior surface in articular bearing contact with the inferior surface of the femoral component. Movement of the bearing on the tibial component is controlled by a control arm. More particularly, the bearing includes a groove that extends in an anterior-posterior direction in the inferior surface of the bearing. A control arm assembly is pivotally mounted to the tibial component and includes an arm that is slidably engaged in the groove of the bearing. Thus, the bearing and the control arm can rotate together on the superior surface of the tibial component. Additionally, the bearing can slide on the superior surface of the bearing and along the arm of the control arm assembly.
The ability of the tibia to move forward relative to the femur is critical in the achievement of maximum passive flexion. If the tibia does not so move its posterior aspect will impinge sooner against the posterior aspect of the femur, thereby limiting flexion sooner. Where the posterior cruciate is not salvageable, or viable, the posterior stabilized knee device shown in U.S. Pat. No. 6,491,726 produces such rearward motion. Where a viable posterior ligament is present one can use this ligament to generate this posterior motion of the femur on the tibia (rollback).
A knee device that allows anterior-posterior motion of the femur on the tibia can allow maximum passive flexion even in the absence of a competent posterior cruciate ligament. As the leg is forced into maximum passive flexion the proximal tibia will be forced forward by pivoting on the impinging proximal, posterior tibial soft tissue if the prosthetic knee allows anterior motion of the proximal tibia. The absence of a competent posterior ligament, coupled with a device that permits anterior-posterior motion of the femur on the tibia, unfortunately, results in anterior-posterior instability of the knee. If this motion is unconstrained, except by the action of functioning ligaments, then the instability is likewise unconstrained and is undesirable.
The position of the tibia during maximum passive flexion activities typically requires substantial axial rotation of the tibia relative to the femur. This rotation (approximately 25°) may be sufficient to produce placement of one of the posterior femoral condyles to be anterior to the posterior edge of its corresponding tibial condyle. That is, the femoral condyle may overhang the tibia on one side. Thus a knee replacement should also allow such rotation, but preferably without overhang. A device where the bearing can rotate on the tibial component is ideal for such a situation.
The prosthesis shown in U.S. Pat. No. 5,702,466 can be used for a knee device to exploit the ability of the posterior cruciate ligament to produce rollback and to provide anterior-posterior translation and axial rotation needed to obtain maximum passive flexion. Unfortunately there have been some problems experienced with this design in clinical use. Anterior knee pain, particularly on flexion, is one of these problems. This probably results from an incompetent posterior cruciate ligament producing anterior motion of the femur on the tibia rather than rollback. This anterior motion will produce impingement between the anterior aspect of the bearing and soft tissue structures of the knee. Such impingement can produce such pain. This incompetence is quite common and is the reason that anterior motion of the femur relative to the tibia is commonly observed with knee designs that allow such motion.
A posterior stabilized knee, as shown in U.S. Pat. No. 6,475,241 or U.S. Pat. No. 6,491,726 is preferred for those situations where a competent posterior ligament is not present. More particularly, the designs shown in U.S. Pat. No. 6,475,241 and U.S. Pat. No. 6,491,726 reliably produce needed rollback and provided needed axial bearing rotation. Further, these designs limit anterior-posterior instability to essentially normal limits. Where there is a competent posterior cruciate ligament a prosthetic device of the type shown in U.S. Pat. No. 5,702,466 seems preferable since it allows the natural structures to provide such action rather than the mechanical structures of the posterior stabilized device.
The problem however is that the identification of a viable cruciate ligament is not easily accomplished by many surgeons and a once competent ligament may become incompetent. Thus it is desirable to improve the performance of the prosthesis shown in U.S. Pat. No. 5,702,466 in the presence of an incompetent posterior cruciate ligament.
FIGS. 11–13 of U.S. Pat. No. 5,702,466 show an embodiment where the arm of the control arm assembly is formed with a channel and where the bearing includes a shoulder engaged in the channel. The channel and the shoulder function to limit anterior movement of the bearing relative to the control arm and the tibial component and, hence, enhance stability in those situations where there is not a viable cruciate ligament or where the ligament becomes incompetent after implantation of the prosthesis. However, the interengageable channel and shoulder complicate implantation of the prosthesis and complicate removal of the prosthesis that may be required intraoperatively or during revision surgery.
Surgery to implant the prosthetic device shown in FIGS. 11–13 of U.S. Pat. No. 5,702,466 typically is completed by resecting the superior end of the tibia and the inferior end of the femur. The resected ends of the tibia and femur may be prepared further by forming cavities. The stem of the tibial component then is inserted into the cavity formed in the resected superior end of the tibia so that the platform of the tibial component is supported on the resected end of the tibia. The bearing then is assembled with the control arm and the cone that projects from the control arm is inserted into the conical recess in the tibial component. The femoral component then is mounted to the resected inferior surface of the femur. This sequence is required because the subassembly of the control arm and the bearing cannot be mounted easily into the conical recess of the tibial component once the femoral component has been mounted to the femur.
Revision surgery occasionally is necessary. One possible reason for revision surgery would be to replace a defective bearing. In this situation, the femoral component is likely to be properly implanted and perfectly functional. The presence of the properly implanted femoral component significantly complicates the revision surgery, particularly during the implantation of the new bearing and control arm assembly. This implantation is particularly impeded for those prostheses where the control arm assembly is formed with a channel and where the bearing includes a shoulder to engage the channel as depicted in FIGS. 11–13 of U.S. Pat. No. 5,702,466. Surgeons may try to retract the joint sufficiently so that the cone of the bearing/control arm subassembly can be inserted into the recess of the tibial component. However, such excessive retraction of the joint can stretch ligaments and complicate post-surgery recovery. In other instances, the surgeon may remove a properly implanted and perfectly functional femoral component so that the components of the prosthesis can be implanted during revision surgery in the same sequence employed during the initial surgery to implant the prosthesis. The femoral component often is secured in place by adhesive, bone tissue or some combination thereof. Hence, the removal of the properly implanted femoral component can damage the femur and contribute to post-surgery trauma for the patient.
The presence of the properly implanted femoral component also can complicate the removal of the bearing and control arm assembly during revision surgery for those instances where the arm of the control arm assembly is formed with a channel and where the bearing includes a shoulder engaged in the channel. In particular, the control arm must be removed with the bearing. However, the cone of the control arm is trapped in the recess of the tibial component. Problems of removing the bearing during revision surgery are less severe than problems relating to the implantation of a new bearing during revision surgery. In particular, the previously implanted bearing can be broken by the surgeon and removed in pieces. This solution is not ideal, but may be acceptable during the bearing-removal phase of revision surgery. However, this option is not available to implant a new bearing because the preferred new bearing is of unitary construction.
The subject invention was developed in view of these problems encountered during revision surgery. An object of the invention is to facilitate proper positioning of a bearing/control arm subassembly during revision surgery and particularly for those prosthetic joints that have structure for limiting anterior movement of the bearing relative to the control arm.